overconsolidated parameters in earth dam

[carmoch]

Hey guys, I have a little problem. I am studying a case of a earth dam which were constructed two years ago(without filling water until now), and I have to make the stability anaylisis;firstly I carry out several boreholes and piezocones up the fill. This is formed with CH clays (it seems not well compacted), and ground is formed with CH-CL clays (I think it is lightly overconsolidated before the fill). The question is that I haven´t (reliable) oedometric and stifness parameteres before of the construction.
Could I suppose that my parameters are overconsolidated if the fill is two years old and it seems that it hasn´t consolidation (with a little sand level lens 10 m deep and there isn´t ground improvement). Which model is better(Mohr Coulomb or hardening soil) for a finite element calculation?

Thank you very much and sorry for my english.

 

[AussieGeoEng]

I assume you are referring to the placed fill? If so, are the construction QA results and previous design report?

If the fill was engineered previously (i.e. placed in 300mm layers and compacted with a pad foot) then I would estimate that the effective vertical pressure (p`c) around 200kPa. Therefore for anything shallower than about 10m OC, deeper than 10m NC.

Is the sand lense in the fill? This may be a horizontal filter.

 

[dgillette]

Assume that it's overconsolidated? No. Having recently built this fill on a lightly OC foundation, you may have caused effective overburden higher than the maximum past pressure, in which case it is normally consolidated, regardless of its age, unless you did it 1000 years ago so aging has caused apparent overconsolidation.

AussieGeoEng's estimate of 200 kPa for the apparent preconsolidation pressure of properly compacted fill is reasonable. However, you have stated that the fill was not well compacted, which makes it difficult to say anything more without oedometer tests, or better, a good set of undrained triaxial tests at the proper densities.

Good luck to you.

 

[ntschwanz]

Because you discuss using the M-C and hardening soil models it seems you are running some type of FEM analysis. Based on what you describe you probably don't have sufficient information to say anything about displacements so why not use a limit equilibrium approach? If you care only about factor of safety the LE analysis should be fine and if you still want to pursue a FEM analysis to find factor of safety (using the c-phi reduction technique) you really don't need to know anything about the modulus values so a M-C approach would be adequate. Of course you still need to properly characterize the problem as with any slope stability analysis.